The use of membranes in various forms, such as sheets and hollow fibers, for the separation of fluids to obtain a desired portion of the fluid is well known. Generally, one or several components of the feed fluid permeate through the membrane and are collected as the permeate portion. The portion of the fluid that cannot pass through the membrane, the retentate, is retained and discharged as fresh portions of the fluid to be separated are supplied to the membrane.
Membrane separation modules are of two different types; hollow fiber and sheet membrane. The modules of the present invention are of the latter type. To date, the two most common configurations for sheet membrane modules have been of the plate and frame type or of the spiral wrap type.
In the plate and frame configuration, a sheet of membrane material is compressed between two rigid frames which hold it flat, provide support against the differential fluid pressure, and provide fluid flow ports to direct the fluid streams across the membrane surface. The plate and frame design requires a large number of components, with commensurate costs, per unit of membrane area. Sealing the frames against the membranes to achieve a tight seal without damaging the membrane is a problem.
U.S. Pat. No. 3,684,097, for instance, provides a plate and frame device for oxygenating or dialyzing blood which includes a pair of frames having rectangular openings and a gas permeable membrane on each face. A plurality of frames are stacked upon each other in substantial contact to define a thin blood film flow space therebetween.
U.S. Pat. No. 4,115,274 is directed toward a reverse osmosis desalinating apparatus which employs a series of porous discs, each covered on both sides by a membrane. The discs are compressed at their edges between pairs of plates which are arranged in stacks. Alternate module plates in the stack are rotated through 180.degree. to provide a zig-zag flow path. Desalinated water enters the porous discs by reverse osmosis and flows radially outward for collection.
U.S. Pat. No. 4,735,718 is directed toward a multilayer membrane separator for the filtration of liquids. The separator provides at least two membrane units, each unit having first and second membrane sheet layers adhered to a filtrate spacer layer. First and second retentate spacers are in direct contact with first and second membranes, respectively and provide a plurality of channels for passage of filtrate and retentate.
In the spiral wrap configuration, a sheet of porous support material is enclosed is a long sleeve of sheet membrane. The sleeve is typically formed by folding a web of membrane over a web of porous support material then sealing the two edges of the membrane. The long section of sleeve is then rolled into a spiral so that a fluid to be treated can flow from one edge of the spiral to the other in the axial direction. The permeating fluid flows within the spiral sleeve and is collected by a permeate manifold at the center of the spiral. The spiral wrap design requires the permeate to flow around inside the spiral sleeve and through the porous support material for considerable distances. This results in performance penalties caused by a significant permeate pressure drop. The seal between the end of the membrane sleeve and the permeate discharge manifold is difficult to make reliably. The end of the membrane sleeve must be sealed and both the sleeve and the side seals are unprotected in the feel fluid stream. These sheet-to-sheet seals must remain tight for the life of the module. Membrane damage during the wrapping process and during subsequent shifting of the wrap are potential problems.
While the aforementioned art discloses some of the way in which sheets of semipermeable membranes may be employed in a separator apparatus, it is nonetheless desirable to provide a novel wafer element providing a plurality of sheet membrane layers oriented transverse of the wafer, as well as a separation module, employing a plurality of such wafers in which the seals between adjacent wafers in the stack are maintained.